Adapter Design Pattern in Java: Before and after

Before

Because the interface between Line and Rectangle objects is incapatible, the user has to recover the type of each shape and manually supply the correct arguments.

class LegacyLine
{
    public void draw(int x1, int y1, int x2, int y2)
    {
        System.out.println("line from (" + x1 + ',' + y1 + ") to (" + x2 + ',' 
          + y2 + ')');
    }
}

class LegacyRectangle
{
    public void draw(int x, int y, int w, int h)
    {
        System.out.println("rectangle at (" + x + ',' + y + ") with width " + w
          + " and height " + h);
    }
}

public class AdapterDemo
{
    public static void main(String[] args)
    {
        Object[] shapes = 
        {
            new LegacyLine(), new LegacyRectangle()
        };
        // A begin and end point from a graphical editor
        int x1 = 10, y1 = 20;
        int x2 = 30, y2 = 60;
        for (int i = 0; i < shapes.length; ++i)
          if (shapes[i].getClass().getName().equals("LegacyLine"))
            ((LegacyLine)shapes[i]).draw(x1, y1, x2, y2);
          else if (shapes[i].getClass().getName().equals("LegacyRectangle"))
            ((LegacyRectangle)shapes[i]).draw(Math.min(x1, x2), Math.min(y1, y2)
              , Math.abs(x2 - x1), Math.abs(y2 - y1));
    }
}

After

The Adapter’s “extra level of indirection” takes care of mapping a user-friendly common interface to legacy-specific peculiar interfaces.

class LegacyLine
{
    public void draw(int x1, int y1, int x2, int y2)
    {
        System.out.println("line from (" + x1 + ',' + y1 + ") to (" + x2 + ',' 
          + y2 + ')');
    }
}

class LegacyRectangle
{
    public void draw(int x, int y, int w, int h)
    {
        System.out.println("rectangle at (" + x + ',' + y + ") with width " + w
          + " and height " + h);
    }
}

interface Shape
{
  void draw(int x1, int y1, int x2, int y2);
}

class Line implements Shape
{
    private LegacyLine adaptee = new LegacyLine();
    public void draw(int x1, int y1, int x2, int y2)
    {
        adaptee.draw(x1, y1, x2, y2);
    }
}

class Rectangle implements Shape
{
    private LegacyRectangle adaptee = new LegacyRectangle();
    public void draw(int x1, int y1, int x2, int y2)
    {
        adaptee.draw(Math.min(x1, x2), Math.min(y1, y2), Math.abs(x2 - x1),
          Math.abs(y2 - y1));
    }
}

public class AdapterDemo
{
    public static void main(String[] args)
    {
        Shape[] shapes = 
        {
            new Line(), new Rectangle()
        };
        // A begin and end point from a graphical editor
        int x1 = 10, y1 = 20;
        int x2 = 30, y2 = 60;
        for (int i = 0; i < shapes.length; ++i)
          shapes[i].draw(x1, y1, x2, y2);
    }
}